I would rebuild the entire coliseum from the ground up and put a "reset" button on every single dragon who is higher than level 1 with a one-time use that sets that dragon back to base breed stats and takes off all of their ability stones without destroying them.

First, I would boost the base damage of caster attacks. Second, I would give casters an attack ability equivalent in effectiveness to Eliminate. Thirdly, I'd fucking fix Reflect.